This invention relates to electrostatographic reproduction machines, and more particularly to an electrostatographic reproduction machine including an intra-job automatic variable machine speed control method and apparatus.
Generally, the process of electrostatographic reproduction includes charging a photoconductive member to a substantially uniform potential so as to sensitize the surface thereof. A charged portion of the photoconductive surface is exposed, for example digitally, at an exposure station to a light image of an original document to be reproduced. Typically, an original document to be reproduced is placed in registration, either manually or by means of an automatic document handler (ADH), on a platen for such exposure. In general, the exposure station includes a means, such as a raster input scanner (RIS) device in a digital machine, for electronically and digitally capturing image information by scanning the document on the platen. When scanning a multicolor original image, the RIS device produces digitized video signals corresponding to color separated images of the original image. Digital machines also each include a raster output scanner (ROS) device for writing or exposing onto the charged photoreceptor, the RIS electronically or digitally captured image.
Exposing an image of an original document as such at the exposure station, records an electrostatic latent image of the original image onto the photoconductive member. The recorded latent image is subsequently developed using a development apparatus by bringing a charged dry or liquid developer material into contact with the latent image. Two-component and single component developer materials are commonly used. A typical two-component dry developer material has magnetic carrier granules with fusible toner particles adhering triobelectrically thereto. A single component dry developer material typically comprising toner particles only can also be used. The toner image formed by such development is subsequently transferred at a transfer station onto a copy sheet fed to such transfer station, and on which the toner particles image is then heated and permanently fused so as to form a "hardcopy" or finished copy of the original image. The finished copy of each original document is then fed to an output tray for subsequent removal and use by an operator.
Development or application of toner particles as above typically depletes toner particles at some rate from developer material in a development unit of the machine. Different jobs of several documents being reproduced, will cause toner depletion at different rates depending on the copy sheet area coverage level by toner particles. In a machine using two component developer material, such depletion undesirably changes the concentration of such particles in the developer material. In order to maintain the concentration of toner particles within the developer material (so as to insure the quality of subsequent images), the depleted toner particles must be replenished at some fixed or variable rate with fresh toner particles. Such fresh toner particles must then be admixed with the carrier particles in order to properly charge them triboeletrically. Unfortunately, there are jobs or sequences in jobs wherein the toner usage or depletion rate far exceeds the fresh toner replenishment and admix rate, ordinarily resulting in poor quality reproductions or machine stoppage and recovery time.
A similar problem is also associated with fusing temperature maintenance wherein a "droop" or "sag" in fusing temperature due to any of several factors, ordinarily too will result in poor quality reproductions. Conventional attempts to avoid such poor quality reproductions have typically involved temporarily suspending the imaging process in order to give the replenishment system time to recover. This is usually the approach because reproduction machines typically have a fixed speed or machine speed.
Examples of references forming a background for the measuring or sensing aspect of the present invention include the following patents. U.S. Pat. No. 5,204,698 discloses a laser printer in which a latent image is generated on a circulating imaging member in accordance with digital image signals and subsequently developed with toner, the number of pixels to be toned is used as an indication of the rate at which toner is being depleted from the developer mixture. The device for dispensing fresh toner to the developer mixture is operated in dependence on the number of pixels to be toned so that there is a pre-established relationship between the pixel count and the length of time for which the dispensing device is in operation. If the efficiency of the dispensing device falls, the pre-established relationship is adjusted so that the toner density in the developed images remains constant. If a predetermined level of adjustment is reached, it is taken as an indication that the supply of toner in the printer is low, and should be replenished.
U.S. Pat. No. 5,349,377 issued Sep. 20, 1994, discloses a process known as pixel counting. The toner usage per copy depends upon primarily the percent of the copy that is covered by toner and the density of the covered area. In electrophotographic printers where the document is scanned, commonly known as scanning printers, the areas or pixels, which represent portions of the text, may be used as an indicator of the amount of toner to be used. This pixel counting system is used in conjunction with an algorithm to determine the amount of toner used per page being printed and, subtracting the used toner from the amount of toner in a full container, determines the current toner level. From this current toner level, a toner low warning is presented to the operator.